1. Field of the Invention
The present invention relates to a process and a catalyst for the catalytic oxidative dehydrogenation of alkylaromatics and paraffin hydrocarbons to give the corresponding alkenylaromatics and olefins, preferably of ethylbenzene to give styrene, water being formed and the dehydrogenation taking place in the absence of free oxidizing agent (ie. oxidizing agent added continuously to the stream of starting materials), such as molecular oxygen or oxygen-containing gases and the redox catalyst consisting of at least one reducible metal oxide being the sole oxygen source and performing the function of an oxygen store or oxygen carrier.
Olefins and alkenylbenzenes, in particular styrene and divinylbenzene, are important monomers for engineering plastics and are produced in large amounts. Styrene is prepared predominantly by nonoxidative dehydrogenation of ethylbenzene over a modified iron oxide catalyst, one mole of hydrogen being formed per mole of styrene. Unfortunately, this is an equilibrium reaction which is carried out at high temperatures of, typically, from 600 to 700.degree. C. and takes place with the conversion of about 60% at a styrene selectivity of about 90%.
The equilibrium can be overcome and a virtually quantitative conversion achieved by oxidative dehydrogenation in which an oxidizing agent, such as molecular oxygen or an oxygen-containing as, is introduced into the stream of starting materials, since water is now formed as an accompanying product. Furthermore, lower reaction temperatures are required for this reaction. However, the disadvantage of this process is the loss of selectivity with respect to the desired product owing to the presence of oxygen, since the high oxygen concentration in the reaction zone promotes total oxidation as a secondary reaction.
It is therefore being proposed to use, as the oxygen carrier, a catalyst consisting of a reducible metal oxide instead of free oxidizing agent (oxygen). The catalyst (simultaneously an oxygen carrier) is gradually consumed and must be regenerated in a second step, restoring the initial activity. In the regeneration phase, for example, coke deposits can also be burnt off. The regeneration is highly exothermic so that the waste heat liberated can be used, for example, for generating steam.
By decoupling the reduction step and oxidation step, the selectivity can be substantially increased.
Two variants are available for the technical realization of this proposal, ie. the separation of the two steps in terms of space and in terms of time.
In the former, a moving bed or a circulating fluidized bed is used so that, after the resulting reaction products have been separated off, the catalyst particles are transported from the dehydrogenation zone to a separate regeneration reactor, in which the reoxidation is carried out. The regenerated catalyst is recycled to the dehydrogenation zone. The catalyst is exposed to high mechanical stresses and must therefore have sufficient hardness.
2. Description of the Related Art
The embodiment using a fixed bed involves periodic switching between the starting material feed and, if necessary after a flushing phase, the regeneration gas.
This principle of separation of the two steps of the redox reaction using a reducible and regeneratable catalyst was first described for the oxidation or ammoxidation of propene to acrolein and acrylic acid or acrylonitrile, respectively (GB 885422; GB 999629; K. Aykan, J. Catal. 12 (1968), 281-190), arsenate and molybdate catalysts being used. The use of the process in the oxidative dehydrogenation of aliphatic alkanes to mono- and diolefins using ferrite catalysts (e.g. U.S. Pat. No. 3,440,299, DE 21 18 344, DE 17 93 499) is likewise known, as is the use for the oxidative coupling of methane to give higher hydrocarbons, different catalyst classes being used (eg. U.S. Pat. No. 4,795,849, DE 3 586 769 with Mn/Mg/Si oxides; U.S. Pat. No. 4,568,789 with Ru oxide; EP 254 423 with Mn/B oxides on MgO; GB 2 156 842 with Mn.sub.3 O.sub.4 spinels). The dehydrodimerization of toluene to stilbene in the absence of free oxygen by means of reducible catalysts, such as Bi/In/Ag oxides (EP 30 837) is also known. Finally, the principle is also applied to the dehydrogenation, dehydrocyclization and dehydroaromatization of paraffin hydrocarbons for gasoline refinement (U.S. Pat. No. 4,396,537 with Co/P oxide catalysts).
EP 397 637 and 403 462 disclose using the principle of the process for the oxidative dehydrogenation of paraffin hydrocarbons and alkylaromatics. According to these publications, reducible metal oxides selected from the group consisting of V, Cr, Mn, Fe, Co, Pb, Bi, Mo, U and Sn are applied to carriers comprising clays, zeolites and oxides of Ti, Zr, Zn, Th, Mg, Ca, Ba, Si and Al and used. V/MgO is particularly preferred.
Although a high yield can be obtained with these catalysts, very pronounced gasification (total combustion) occurs in the initial phase of the dehydrogenation when the hydrocarbon comes into contact with the freshly regenerated and therefore particularly active catalyst. Apart from the loss of raw materials, in addition the amount of oxygen consumed is considerably more than for the straightforward dehydrogenation, so that a large part of the oxygen carrier is prematurely exhausted and the cycle times unnecessarily become shorter.